Each manufactured article is normally provided with a coating. There are anticorrosive coatings which protect articles from the effects of aggressive media. There are special-purpose coatings which impart certain mechanical or electric properties to an article, such as current-carrying properties, a certain friction coefficient, increased hardness, a certain degree of thermal conductivity, etc. Finally, there are decorative coatings intended to give an article an attractive appearance. The quality of an article is largely determined by the thickness and uniformity of the coating, wherefore each coating application method necessitates rapid coating thickness measurements preferably taken right in the course of the production process. Non-destructive checking of the thickness of coatings makes it possible to improve a number of specific properties of coatings and reduce excessive consumption of coating materials, especially of precious metals and rare elements, which accounts for the fact that thickness measurements become part and parcel of coating application processes. Coating thickness measurements must go hand in hand with data processing and allow automation of the measuring process.
There are a number of different types of thickness gauges used to measure the thickness of coatings. These include electromagnetic, magnetic, radioisotope, ultrasonic, and other types of gauges. Electromagnetic and radioisotope thickness gauges are the commonest. The field of application of radioisotope thickness gauges is limited by the presence of a radioisotope source which requires special skills and precautions on the part of the personnel. Another limitation lies in the fact that there is a specific relationship, i.e. a specific calibration curve for each coating-base combination. As to the development of electromagnetic methods, there have been two basic trends in this field in recent years. The first trend is the development of new types of thickness gauges and new data conversion techniques, as well as improvement of the existing data conversion techniques. The second trend is automation of measuring processes.
Despite a variety of engineering approaches, the existing devices for measuring the thickness of coatings have been unable to meet the stringent accuracy requirements imposed by the industry. Almost all the existing thickness gauges feature a non-linear dependence between the readings of the instrument and the thickness being measured. As a result, a thickness gauge must be provided with nonlinear scales in a number corresponding to that of measurement ranges. This is true of such instruments as M -10 (cf. V. V. Kluyev et al., Electromagnetic Instrument for Measuring the Thickness of Nonferromagnetic Coatings on Ferromagnetic Bases, Defectoscopia, No. 6, 1971, 118); the above consideration applies to nondestructive inspection instruments of the MT-20H, MT-30H and BT-30H models developed at the Institute of Introscopy in the Soviet Union (cf. Zavodskaya laboratoriya, No. 6, 1974, p. 761); the above consideration also applies to such thickness gauges as ULTRAMETR A-9 and A-51 manufactured in Poland (cf. St. Sekowski, Nowe warstwomierze type A-9, A-51, Mechanik, No. 5, 1974, p. 306), PERMASCOPE and ISOSCOPE manufactured by Helmut Fischer of the Federal Republic of Germany and ELEKTROTEST and MONIMETR manufactured by Institut Dr. F/orster of the Federal Republic of Germany (cf. Monimeter 2,094, Maschine und Werkzeug, No. 13, 1975, 76, p. 32). The foregoing disadvantage accounts for limited application of the existing types of thickness gauges. Another disadvantage of the conventional thickness gauges is a varying degree of accuracy in different measurement ranges; in fact, there may be variations in the accuracy even within a single measurement range.
The latter disadvantage is partially eliminated in the thickness gauges of the GIL-41 model manufactured in Poland and MICRO-DERM MD-3 model manufactured by UPA of the United States. These thickness gauges employ sets of interchangeable scales. Scales for different measurement ranges are interchangeable strips placed on the needle indicator of the instrument. The use of interchangeable scales involves a number of difficulties and does not make it possible to automate measuring processes because each scale must correspond to certain base and coating standards. Digital thickness gauges, for example, the gauge of the MT-40H type (cf. A. L. Dorofeyev and G. A. Lyubashov, "The Use of Eddy Currents for Coating Thickness Measurements", Machinostroyeniye Publishers, Moscow, 1975, in Russian) employ a highly sophisticated data conversion system, based on time-pulse conversion of signals; despite the complexity of this system, the measurement process is automated only to a limited extent. Most of the existing instruments necessitate a preliminary measurement of the coating thickness so as to select the measurement range, which requirement further impedes automation of measuring processes.
There is known a device for measuring the thickness of coatings, wherein an electric signal generator is connected to a thickness gauge which is electrically coupled to a conversion circuit intended to convert an electric signal at the output of the thickness gauge to an electric signal of a magnitude proportional to the thickness of the coating being measured, the conversion circuit comprising, in turn, a reference voltage forming unit electrically coupled to a subtractor and connected to a recorder (cf. the above-mentioned M -10 model).
The device under review is intended for measuring the thickness of current-carrying nonmagnetic coatings of such materials as copper, zinc, tin and chromium, and of dielectric nonmagnetic coating of such materials as lacquers, varnishes, films and paints, applied onto articles of ferro-magnetic materials. The operating principle of the device is based upon measuring the permeance over the gauge-ferro-magnetic base (article) portion. Output voltage of the thickness gauge is a function of the distance between the gauge and article and is indicative of the coating thickness. The conversion circuit of the device under review further includes an auxiliary amplifier, a two-stage amplifier and an emitter follower. The subtractor is a gain-phase detector.
From the output of the thickness gauge, voltage is applied via the amplifier to one of the inputs of the subtractor. A signal corresponding to the selected measurement range is applied from the reference voltage forming unit to the second input of the subtractor. Subtraction is performed, and the difference signal is sent to the recorder. The desired measurement range is selected by the operator by switching the reference voltage and appropriately adjusting the gain.
As the instruments discussed above, this device, too, has a number of disadvantages. These include a nonlinear dependence between the readings of the thickness gauge and the thickness of the coating being measured; as a result, there are four nonlinear measurement ranges, which accounts for variations in the measuring sensitivity from range to range. A serious inconvenience is the necessity of a preliminary rough estimation of the thickness to be measured and an appropriate selection of a measurement range, as well as the necessity of adjusting the instrument for each working range with reference to a number of points. These requirements completely rule out automation of the measuring process. The presence of the gain-phase detector makes readings of the instrument dependent upon phase oscillation.